How to farm heart scales?

What’s the best way to farm heart scales? I’m before the 3rd gym. I tried the midna mine luvdisc method, but I’ve gotten 1 in over 20 minutes, so if there’s a faster way, I’d appreciate it

Get a secret base, visit 10 bases and take their flags. It’ll take you around 10 minutes, and you’ll get 3 heart scales.
[Flag Guy Prizes]

This heart scale guide should help.